The use of liquid carbon dioxide (CO2) as a fracture fluid for stimulating oil and gas containing formations is well known in the art. In comparison to water, liquid CO2 is non-damaging to the formations and flows back readily after fracture treatment. A critical aspect of any fracture treatment is the addition of proppant to the fracture fluid. This particulate matter props open the fractures allowing the oil or gas to flow into the wellbore in the production phase.
When liquid CO2 is used as the fracture fluid, proppant must be added to the liquid CO2 under pressure, since CO2 does not exist as a liquid below its triple point pressure of approximately 60 psig. This is conveniently achieved in a batch blender system such as the one described in U.S. Pat. No. 4,374,545 to Bullen et al, and may be performed at pressures in the range 150 to 400 psig, for example. In such a system, proppant is filled into the batch blender followed by liquid CO2. During fracture treatment, the now cooled proppant is metered out of the batch blender into a flowing liquid CO2 fracture stream and the resultant proppant-laden liquid CO2 stream is pumped to high pressure (e.g. 2,000 to 10,000 psig), prior to being injected into the wellbore. Limitations of this type of batch blender include the amount of proppant that can be loaded into the batch blender, approximately 20 tons in Bullen et al., and the relative difficulty in being able to change proppant as desired during the fracture treatment. For example, oftentimes it is preferred to use finer proppants to better fill the fracture tips and coarser proppants, having higher conductivity, to fill the body of the fracture.
The capacity limitation of these batch blenders can be alleviated by utilizing a number of blenders together, increasing the capacity of each blender, or by attempting to refill empty blenders during the fracture treatment. However, it will be appreciated that this requires significant additional capital and process complexity, and that the total proppant capacity may still be limited, especially when compared to larger fracture treatments (e.g., in excess of 500 tons of proppant per well), which is common.
The difficulty in being able to change proppants during a fracture treatment arises from the fact that only one type of proppant is usually preloaded into this type of batch blender and, therefore, more than one batch blender will be required if it is desired to utilize more than one type of proppant. The batch blender may be compartmentalized to accept multiple types of proppant, but this adds complexity and has limited job-to-job flexibility.
In conventional fracture treatments utilizing aqueous based fracture fluids, proppant, and other additives, are added to the base fracture fluid at atmospheric pressure in a continuous-style blender. This blender is herein termed continuous, as, within the limitations of on-site material supply and equipment reliability etc., it can continuously input feed streams and continuously output a product stream. These blenders typically also have a low hold-up, or inventory, of material in the blender which means that changes to the input streams are rapidly seen in the output stream. In this way it is possible to change the input proppant stream as necessary and thereby quickly change the type of proppant being utilized in the fracture treatment, and also to supply larger quantities of proppant to the fracture treatment by virtue of the continuous nature of these blenders.
By way of example, FIG. 1 illustrates a related art batch proppant system that also incorporates a polymer and a co-solvent. In this example, proppant is stored in batch blender vessel 130 together with liquid CO2, and is metered from batch blender vessel 130 via conduit 131 into low pressure liquid CO2 stream 111. Proppant-laden liquid CO2 is then pumped to high pressure by fracture pump(s) 140, and passes to the wellhead via main conduit 141. In the case where friction reduction or viscosification of the liquid CO2 is required for example, secondary liquid (co-solvent) and polymer are added via conduits 241 and 341, respectively, the secondary liquid being required to enable dissolution of the polymer into the liquid CO2. The quantity of proppant added is limited by the capacity of vessel 130, and the proppant type may not readily be changed as desired during the treatment.
Further limitations of liquid CO2 for use as a fracturing fluid include high pressure losses under high velocity conditions, and inherent low viscosity. Low pressure losses are desired in the wellbore, or tubing inserted therein, to enable higher fluid flow rates or minimize hydraulic horsepower requirements, while higher viscosity is desired to more effectively transport and place proppant in the fracture. Various types of high molecular weight polymers are commonly used in conventional aqueous-based fracturing fluids to achieve these two objectives. However liquid CO2, on its own, is found to be a relatively poor solvent for high molecular weight polymers, even when raised to fracturing pressures (e.g., 2,000 to 10,000 psig). It is, therefore, required that a co-solvent is added to the liquid CO2 at a sufficient concentration to facilitate dissolution of specific high molecular weight polymers, and thereby achieve friction reduction or viscosification.
Thus, to overcome the disadvantage in the related art, one of the objects of the present invention is to add proppant to a secondary liquid (co-solvent) at approximately atmospheric pressure whereupon it is added and/or mixed with the primary high pressure liquid CO2 stream and polymer directly at the wellhead or at the very least downstream of the high pressure fracturing pumps.
It is another object of the invention to be able to provide the addition of proppant to the secondary liquid continuously, lending the ability to change the proppant type as desired during the fracture treatment, and to be able to continuously add proppant to the fracture fluid during the fracture treatment, or as desired.
It is a further object of the invention to be able to reduce frictional losses or viscosify the fracturing fluid through addition of a suitable polymer to the liquid CO2 and secondary liquid (co-solvent) stream.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.